Pervaporation is a membrane-based process used to separate solutions on the basis of differences in the volatilities or diffusion characteristics of the components. A liquid mixture contacts one side of a membrane; the permeate is removed as a vapor from the other side. Transport through the membrane is induced by the difference in partial pressure between the liquid feed solution and the permeate vapor. This partial-pressure difference can be maintained in several ways, such as drawing a vacuum on the permeate side of the system, sweeping the permeate side with a carrier gas, or simply cooling the permeate vapor, causing it to condense.
In any membrane process, the membranes must be packaged. Spiral-wound modules, developed for reverse osmosis and now used for reverse osmosis, ultrafiltration and some gas separation applications, are advantageous because they pack very large areas of membrane into a small volume. Spiral-wound modules work well in processes characterized by high pressure differentials between feed and permeate sides and low product flow volumes. Absent these process attributes, spiral-wound modules run into difficulties, because the inevitable pressure drop along the permeate channel diminishes the driving force available for membrane permeation. The lower the inherent transmembrane driving pressure, the worse this problem becomes. The worst scenario is encountered in processes characterized by a low driving pressure, but a relatively high permeate flow, particularly if the process relies on a high membrane separation factor between the components. In this case, the diminished transmembrane pressure differential resulting from the pressure drop along the permeate channel may not only ruin the flux, but may also spoil the separation properties. Unfortunately, pervaporation is just such a process. Modern pervaporation membranes offer separation factors in the 100s or more for organic components over water, for example, and have high permeate fluxes. Pervaporation is inherently a process that relies on a small difference in partial vapor pressure between the feed and permeate sides of the membrane, a difference that is often as little as 10 cmHg or less.
U.S. Pat. No. 4,789,480 described a spiral-wound pervaporation module in which the permeate channel pressure drop problem is addressed by using a radial-flow configuration. To date, the art has not been successful, to applicant's knowledge, in developing industrial pervaporation systems containing spiral-wound modules. The only successful industrial pervaporation installations use plate-and-frame modules, where the pressure drop problem is not an issue. GFT, of Neunkirchen, West Germany, has installed many pervaporation dehydration systems using this module technology.